{
    "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
    "data": {
        "url": "https://vega.github.io/vega-lite/data/cars.json"
    },
    "description": "Custom-shaped and coloured by origin (should only have a single set of legend items)",
    "encoding": {
        "color": {
            "field": "Origin",
            "type": "nominal"
        },
        "opacity": {
            "value": 0.6
        },
        "shape": {
            "field": "Origin",
            "type": "nominal"
        },
        "x": {
            "field": "Horsepower",
            "type": "quantitative"
        },
        "y": {
            "field": "Miles_per_Gallon",
            "type": "quantitative"
        }
    },
    "mark": {
        "filled": true,
        "type": "point"
    },
    "transform": [
        {
            "as": "YearOfManufacture",
            "calculate": "year(datum.Year)"
        },
        {
            "filter": "datum.YearOfManufacture == 1970"
        }
    ]
}